Systems, methods and apparatus for planar expulsion shields

ABSTRACT

A superconducting magnetic shielding system includes first and second planar superconducting shields respectively positioned above and below an environment to be shielded. The shields are each thermally coupled to a cold source at a respective thermalizing point. When the shields are cooled into the superconducting regime, they passively block magnetic fields via the Meissner Effect. Each shield may also be shaped to produce a smooth temperature gradient extending away from its thermalizing point; thus, as the shields are cooled, magnetic fields may be expelled away from the thermalizing point and, consequently, away from the environment to be shielded. A heater may also be provided opposite the thermalizing points to improve control of the temperature gradient.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit under 35 U.S.C. 119(e) of U.S.Provisional Patent Application Ser. No. 61/589,109, filed Jan. 20, 2012,which is incorporated herein by reference in its entirety.

BACKGROUND Field

The present systems, methods and apparatus generally relate to magneticshielding and particularly relate to shielding a component of a magneticfield using planar expulsion shields.

Superconducting Qubits

There are many different hardware and software approaches underconsideration for use in quantum computers. One hardware approach usesintegrated circuits formed of superconducting materials, such asaluminum or niobium. The technologies and processes involved indesigning and fabricating superconducting integrated circuits aresimilar in some respects to those used for conventional integratedcircuits.

Superconducting qubits are a type of superconducting device that can beincluded in a superconducting integrated circuit. Superconducting qubitscan be separated into several categories depending on the physicalproperty used to encode information. For example, they may be separatedinto charge, flux and phase devices, as discussed in, for exampleMakhlin et al., 2001, Reviews of Modern Physics 73, pp. 357-400. Chargedevices store and manipulate information in the charge states of thedevice, where elementary charges consist of pairs of electrons calledCooper pairs. A Cooper pair has a charge of 2e and consists of twoelectrons bound together by, for example, a phonon interaction. Seee.g., Nielsen and Chuang, Quantum Computation and Quantum Information,Cambridge University Press, Cambridge (2000), pp. 343-345. Flux devicesstore information in a variable related to the magnetic flux throughsome part of the device. Phase devices store information in a variablerelated to the difference in superconducting phase between two regionsof the phase device. Recently, hybrid devices using two or more ofcharge, flux and phase degrees of freedom have been developed. See e.g.,U.S. Pat. No. 6,838,694 and U.S. Pat. No. 7,335,909.

Examples of flux qubits that may be used include rf-SQUIDs, whichinclude a superconducting loop interrupted by one Josephson junction, ora compound junction (where a single Josephson junction is replaced bytwo parallel Josephson junctions), or persistent current qubits, whichinclude a superconducting loop interrupted by three Josephson junctions,and the like. See e.g., Mooij et al., 1999, Science 285, 1036; andOrlando et al., 1999, Phys. Rev. B 60, 15398. Other examples ofsuperconducting qubits can be found, for example, in ll'ichev et al.,2003, Phys. Rev. Lett. 91, 097906; Blatter et al., 2001, Phys. Rev. B63, 174511, and Friedman et al., 2000, Nature 406, 43. In addition,hybrid charge-phase qubits may also be used.

The qubits may include a corresponding local bias device. The local biasdevices may include a metal loop in proximity to a superconducting qubitthat provides an external flux bias to the qubit. The local bias devicemay also include a plurality of Josephson junctions. Eachsuperconducting qubit in the quantum processor may have a correspondinglocal bias device or there may be fewer local bias devices than qubits.In some embodiments, charge-based readout and local bias devices may beused. The readout device(s) may include a plurality of dc-SQUIDmagnetometers, each inductively connected to a different qubit within atopology. The readout device may provide a voltage or current. Thedc-SQUID magnetometers including a loop of superconducting materialinterrupted by at least one Josephson junction are well known in theart.

Quantum Processor

A computer processor may take the form of an analog processor, forinstance a quantum processor such as a superconducting quantumprocessor. A superconducting quantum processor may include a number ofqubits and associated local bias devices, for instance two or moresuperconducting qubits. Further detail and embodiments of exemplaryquantum processors that may be used in conjunction with the presentsystems, methods, and apparatus are described in at least US PatentPublication No. 2006-0225165 (now granted as U.S. Pat. No. 7,533,068),U.S. patent application Ser. No. 12/013,192 (now granted as U.S. Pat.No. 8,195,596), U.S. Provisional Patent Application Ser. No. 60/986,554filed Nov. 8, 2007 and entitled “Systems, Devices and Methods for AnalogProcessing,” and U.S. Provisional Patent Application Ser. No.61/039,710, filed Mar. 26, 2008 and entitled “Systems, Devices, AndMethods For Analog Processing.”

A superconducting quantum processor may include a number of couplingdevices operable to selectively couple respective pairs of qubits.Examples of superconducting coupling devices include rf-SQUIDs anddc-SQUIDs, which couple qubits together by flux. SQUIDs include asuperconducting loop interrupted by one Josephson junction (an rf-SQUID)or two Josephson junctions (a dc-SQUID). The coupling devices may becapable of both ferromagnetic and anti-ferromagnetic coupling, dependingon how the coupling device is being utilized within the interconnectedtopology. In the case of flux coupling, ferromagnetic coupling impliesthat parallel fluxes are energetically favorable and anti-ferromagneticcoupling implies that anti-parallel fluxes are energetically favorable.Alternatively, charge-based coupling devices may also be used. Othercoupling devices can be found, for example, in US Patent Publication No.2006-0147154 (now granted as U.S. Pat. No. 7,619,437) and U.S. patentapplication Ser. No. 12/017,995 (now published as US 2008-0238531 A1).Respective coupling strengths of the coupling devices may be tunedbetween zero and a maximum value, for example, to provide ferromagneticor anti-ferromagnetic coupling between qubits.

Superconducting Processor

A computer processor may take the form of a superconducting processor,where the superconducting processor may not be a quantum processor inthe traditional sense. For instance, some embodiments of asuperconducting processor may not focus on quantum effects such asquantum tunneling, superposition, and entanglement but may ratheroperate by emphasizing different principles, such as for example theprinciples that govern the operation of classical computer processors.However, there may still be certain advantages to the implementation ofsuch superconducting processors. Due to their natural physicalproperties, superconducting processors in general may be capable ofhigher switching speeds and shorter computation times thannon-superconducting processors, and therefore it may be more practicalto solve certain problems on superconducting processors.

Refrigeration

According to the present state of the art, a superconducting materialmay generally only act as a superconductor if it is cooled below acritical temperature that is characteristic of the specific material inquestion. For this reason, those of skill in the art will appreciatethat a computer system that implements superconducting processors mayimplicitly include a refrigeration system for cooling thesuperconducting materials in the system. Systems and methods for suchrefrigeration systems are well known in the art. A dilution refrigeratoris an example of a refrigeration system that is commonly implemented forcooling a superconducting material to a temperature at which it may actas a superconductor. In common practice, the cooling process in adilution refrigerator may use a mixture of at least two isotopes ofhelium (such as helium-3 and helium-4). Full details on the operation oftypical dilution refrigerators may be found in F. Pobell, Matter andMethods at Low Temperatures, Springer-Verlag Second Edition, 1996, pp.120-156. However, those of skill in the art will appreciate that thepresent systems, methods and apparatus are not limited to applicationsinvolving dilution refrigerators, but rather may be applied using anytype of refrigeration system.

BRIEF SUMMARY

A superconducting magnetic shielding system operable to shield a devicefrom a component of a magnetic field, the device having at least a firstsurface and a second surface that is opposite the first surface, may besummarized as including a first planar shield formed of a material thatis superconducting below a critical temperature, the first planar shielddefined by a perimeter and an area that is larger than an area of thedevice; a second planar shield formed of a material that issuperconducting below a critical temperature, the second planar shielddefined by a perimeter and an area that is larger than the area of thedevice, wherein the first planar shield and the second planar shield areparallel with one another, the first planar shield is proximate thefirst surface of the device, the second planar shield is proximate thesecond surface of the device, and wherein the first and second planarshields are both oriented normal to the component of the magnetic field;a first thermally conductive lead that is thermally coupled to a pointproximate the perimeter of the first planar shield and operable tothermally couple to a cold surface; and a second thermally conductivelead that is thermally coupled to a point proximate the perimeter of thesecond planar shield and operable to thermally couple to the coldsurface. The point proximate the perimeter of the first planar shieldmay be aligned with the point proximate the perimeter of the secondplanar shield. The first and second planar shields may be positionedsuch that the device is proximate a center of the first planar shieldand a center of the second planar shield. A shape of the first planarshield may be similar to a shape of the second planar shield. The shapeof the first planar shield may be triangular and the shape of the secondplanar shield may be triangular. The area of the first planar shield maybe similar to the area of the second planar shield.

The device may include a material that is superconducting below acritical temperature. The critical temperature of the first planar maybe higher than the critical temperature of the device and the criticaltemperature of the second planar shield may be higher than the criticaltemperature of the device. The first planar shield and the second planarshield may both be formed of a material selected from the groupconsisting of: niobium, tin, aluminum, lead, bulk niobium-titaniumalloy, or a layer of niobium-titanium alloy deposited onto a substrate.The device may include niobium. The device may be a superconducting chipincluding a superconducting processor. The superconducting processor mayinclude a superconducting quantum processor. The device may be planar.The device may be parallel to both the first planar shield and thesecond planar shield.

A separation distance between the first planar shield and the secondplanar shield may be less than a length of the first planar shield, awidth of the first planar shield, a length of the second planar shield,and a width of the second planar shield. The first thermally conductivelead and the second thermally conductive lead may both comprise copper.A respective critical field of each of the first planar shield and thesecond planar shield may be higher than a maximum magnitude of themagnetic field. The magnetic shielding system may further include acontrollable heating device comprising a first heater and a secondheater, wherein the first heater is proximate a first end of the firstplanar shield and the second heater is proximate a first end of thesecond planar shield. The first end of the first planar shield may be afurthest end from the point proximate the perimeter of the first planarshield at which the first thermally conductive lead is thermallycoupled, and the first end of the second planar shield may be a furthestend from the point proximate the perimeter of the second planar shieldat which the second thermally conductive lead is thermally coupled.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

In the drawings, identical reference numbers identify similar elementsor acts. The sizes and relative positions of elements in the drawingsare not necessarily drawn to scale. For example, the shapes of variouselements and angles are not drawn to scale, and some of these elementsare arbitrarily enlarged and positioned to improve drawing legibility.Further, the particular shapes of the elements as drawn are not intendedto convey any information regarding the actual shape of the particularelements, and have been solely selected for ease of recognition in thedrawings.

FIG. 1 is a perspective view of a magnetic shielding system thatimplements a device enclosed by two planar shields in accordance withthe present systems, methods and apparatus.

FIG. 2 is a side view of a magnetic shielding system in accordance withthe present systems, methods and apparatus.

FIG. 3 is a schematic diagram of a magnetic shielding system thatincludes 2 controllable heaters operable to further reduce magneticfields trapped in the planar shields by actively steepening thetemperature gradient.

FIG. 4 is a top view of a magnetic shielding system showing a controlledtemperature gradient of a planar shield in accordance with the presentsystems, methods and apparatus.

FIG. 5A is an exemplary plot of the temperature along a line bisecting aplanar shield from FIG. 4.

FIG. 5B is a plot of the temperature gradient of FIG. 5A.

DETAILED DESCRIPTION

In the following description, some specific details are included toprovide a thorough understanding of various disclosed embodiments. Oneskilled in the relevant art, however, will recognize that embodimentsmay be practiced without one or more of these specific details, or withother methods, components, materials, etc. In other instances,well-known structures associated with electronics systems, such as powersupplies, signal generators, and control systems includingmicroprocessors and drive circuitry have not been shown or described indetail to avoid unnecessarily obscuring descriptions of the embodimentsof the present systems and methods.

Unless the context requires otherwise, throughout the specification andclaims which follow, the word “comprise” and variations thereof, suchas, “comprises” and “comprising” are to be construed in an open,inclusive sense, that is as “including, but not limited to.”

Reference throughout this specification to “one embodiment,” or “anembodiment,” or “another embodiment” means that a particular referentfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment. Thus, the appearancesof the phrases “in one embodiment,” or “in an embodiment,” or “anotherembodiment” in various places throughout this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures, or characteristics may be combined inany suitable manner in one or more embodiments.

It should be noted that, as used in this specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the content clearly dictates otherwise. Thus, for example,reference to a shielding system including “a magnetic shield” includes asingle magnetic shield, or two or more magnetic shields. It should alsobe noted that the term “or” is generally employed in its sense including“and/or” unless the content clearly dictates otherwise.

The headings provided herein are for convenience only and do notinterpret the scope or meaning of the embodiments.

The various embodiments described herein provide systems, methods, andapparatus for achieving low magnetic fields and/or low magnetic fieldgradients over a particular area or volume. The present systems,methods, and apparatus include both passive and active techniques forreducing a component of a magnetic field and/or a magnetic fieldgradient in an environment.

A common technique for controlling ambient magnetic fields in asensitive system is to encase the system in a hollow superconductingcylinder that is closed at one end. The superconducting cylinder may bewrapped by at least one compensation coil. At a temperature above thecritical temperature of the superconducting cylinder (that is, while thesuperconducting cylinder is not behaving as a superconductor), theambient magnetic field inside the cylinder is monitored with ameasurement device, such as for example a magnetometer. A DC-current ispassed through the compensation coil(s) to produce a magnetic field thatinterferes (either destructively or constructively, depending on thedesired impact on the system) with the ambient magnetic field measuredinside the cylinder. Once the desired magnetic field has been producedand maintained inside the cylinder, the temperature of the system isreduced below the critical temperature of the superconducting cylindersuch that the cylinder becomes superconducting. When this occurs, thecylinder may naturally trap the magnetic flux that is being generated bythe compensation coil(s), thereby locking the state of the field(s)inside the cylinder and allowing the DC-current being applied to thecompensation coil(s) to be switched off. Throughout the remainder ofthis specification, this technique of implementing compensation coilswrapped around a cylindrical superconducting shield is referred to as“the cylinder approach.”

Even with the cylinder approach, the magnetic field cannot to beshielded completely. The uncompensated magnetic field may be dominatedby an unavoidable remnant magnetic field sourced in the cylindricalsuperconducting shield. The present systems, methods and apparatusdescribe a space-efficient, cost-efficient and flexible technique thatmay be used instead of or in addition to the cylinder approach tofurther reduce the ambient magnetic field in a localized environment andimprove shielding performance.

Embodiments of the present systems, methods and apparatus implement atleast two planar shields that are superconducting at or below a criticaltemperature. The planar shields may be triangular in shape and orientednormal to the component of a magnetic field that mostly affects thedevice or system being shielded. Throughout this specification, the term“triangular” is often used to describe the shape of a magnetic shield,e.g., as in “a triangular planar shield.” As described in more detailbelow, a triangular shape may be advantageous for certain applications,but such a shape is used only for exemplary purposes herein and thepresent systems, methods and apparatus may employ any shape of magneticshield, such as rectangular, square, circular, hexagonal, etc.Furthermore, the term “triangular” itself is used loosely to describe an“at least approximately triangular” geometry having three sides and mayemploy, for example, sharp or rounded corners, straight or curved edges,etc. Similarly, the term “planar” is used throughout this specificationand the appended claims to generally describe an object, e.g., “a planarshield” having a substantially flat surface in a two-dimensional planeand a length and a width in the two-dimensional plane that are each,respectively, greater than a thickness of the object in a thirddimension perpendicular to the two-dimensional plane. The term “planar”is used loosely to describe an “at least approximately planar” geometryand may employ, for example, a surface having contours, surfacetextures, a slight curvature, etc.

FIG. 1 is a perspective view of a magnetic shielding system 100 inaccordance with the present systems, methods and apparatus. Magneticshielding system 100 employs two planar shields 110 and 111, each ofwhich is made of superconducting material such as niobium, aluminum,tin, lead, or a superconducting alloy, such as NbTi. Magnetic shieldingsystem 100 may be employed to shield a particular device, such as asuperconducting chip 120. Superconducting chip 120 may be, for example,a superconducting processor chip such as a superconducting quantumprocessor chip. Planar shields 110 and 111 may shield superconductingchip 120 from a magnetic field, and may be oriented normal to thecomponent of the magnetic field that has the largest effect onsuperconducting chip 120. For example, as illustrated in FIG. 1,superconducting chip 120 may be a planar device and thus the componentof the magnetic field that is normal to the planar surface ofsuperconducting chip 120 may be the component of the magnetic field thathas the largest effect on superconducting chip 120. When planar shields110 and 111 are superconducting, any magnetic field that impinges onplanar shields 110 and 111 may bend around planar shields 110 and 111(e.g., in accordance with the Meissner Effect) rather than flowingstraight through. Planar shield 110 is positioned proximate a firstsurface of superconducting chip 120 and planar shield 111 is positionedproximate a second surface of superconducting chip 120, where the secondsurface of superconducting chip 120 is opposite the first surface ofsuperconducting chip 120, thereby sandwiching superconducting chip 120.Thus, an impinging magnetic field that bends around planar shields 110and 111 may also bend around superconducting chip 120. Planar shield 110may have an area similar to the area of planar shield 111, whereassuperconducting chip 120 may have an area no bigger than that of planarshields 110 and 111.

The shielding effect described above (i.e., based on the MeissnerEffect) is a passive form of magnetic shielding. Despite theimplementation of passive magnetic shielding systems, it can beextremely difficult to reduce magnetic fields below a certain point(e.g., to the range of nanoTeslas). In applications where extremely lowfields (e.g., on the order of nanoTeslas or less) are desired, such asin a system employing a superconducting quantum processor chip, it canbe advantageous to introduce a mechanism for dealing with unwantedmagnetic fields that cannot be completely shielded by passive means. Inthe case of a superconducting quantum processor chip, unwanted magneticfields may manifest themselves as unwanted magnetic flux (i.e.,“fluxons”) trapped within at least some of the superconducting deviceson the chip. In U.S. Pat. No. 7,687,938, an on-chip superconductingplane is used to passively shield on-chip devices from unwanted magneticfields originating from either on or off the chip. In US PatentPublication 2011-0237442, trapped fluxons are also moved by introducinga preferential flux gradient and/or a temperature gradient.

Planar shields 110 and 111 are each thermalized at a respective point onor near their perimeter by a cold surface 140 (which is elsewherethermally coupled to a cold source, such as a cryogenic refrigerationsystem—not shown) via thermally conductive leads 150 and 151respectively. Thermally conductive leads 150 and 151 and cold surface140 may be made from thermally conductive material such as copper,brass, gold-plated copper, or gold-plated brass. As system 100 iscooled, planar shields 110 and 111 become superconducting at regionsthereof that are at or below the critical temperature of the materialfrom which planar shields 110 and 111 are formed. Points 160 and 161where planar shields 110 and 111 make contact with thermally conductiveleads 150 and 151, respectively, (hereafter the “thermalizing points”)become superconducting first because these are the points from whichplanar shields 110 and 111 are cooled. The “superconductivity” thenspreads across planar shields 110 and 111 away from thermalizing points160 and 161 as the rest of their respective volumes cool below theircritical temperature. The planarity of planar shields 110 and 111enables smooth transition of temperature along their respective volumesaway from thermalizing points 160 and 161. This transition patterncreates a preferential gradient for magnetic flux away from thermalizingpoints 160 and 161, towards edge 110 a of planar shield 110 and towardsedge 111 a of planar shield 111 which are furthest from thermalizingpoint 160 and 161 respectively. Thus, the controlled cooling of planarshields 110 and 111 naturally expels magnetic flux away fromsuperconducting chip 120, thereby reducing trapped magnetic fluxons insuperconducting chip 120. Superconducting chip 120 may be mounted on asample holder 130 which is thermally coupled to cold surface 140. Thus,sample holder 130 may also be made from a thermally conductive materialsuch as copper, gold, gold-plated copper, or gold-plated brass. Whensample holder 130 cools, superconducting chip 120 cools and becomessuperconducting as the temperature of chip 120 drops below its criticaltemperature. Magnetic fields in the vicinity of chip 120 may be measuredusing a magnetometer 121. In system 100, planar shields 110 and 111 arealigned one above the other. However, in practice the entire system maybe rotated such that shields 110 and 111 are beside rather than aboveand below superconducting chip 120. Thermally conductive lead 150 isconnected to thermalizing point 160 on or near the perimeter of planarshield 110 and thermally conductive lead 151 is connected tothermalizing point 161 on or near the perimeter of planar shield 111such that thermalizing points 160 and 161 are substantially aligned oneon top of the other. Those of skill in the art will appreciate thatplanar shields 110 and 111, and their respective thermalizing points 160and 161, need not be aligned precisely as illustrated in FIG. 1.However, aligning respective thermalizing points 160 and 161 of planarshields 110 and 111 enables planar shields 110 and 111 to cool andbecome superconducting in substantially the same direction, therebyminimizing the magnetic field intensity at the transition boundary as itspreads across the respective surfaces of planar shields 110 and 111during cooling through their respective critical temperatures.

Throughout this specification the term “thermalizing point” is used todescribe the point where each thermally conductive lead is connected totheir respective planar shields. Planar shields 110 and 111 are eachillustrated as being triangular in shape, though as described above anyalternative shape may similarly be employed in accordance with thepresent systems, methods, and apparatus. In some applications, employingtriangular planar shields may be advantageous because the triangularshape provides a negative resistance gradient from a first “thermalizedcorner” (i.e., a thermalizing point located at or near a corner of thetriangular planar shield, such as thermalizing points 160 and 161 in theFigure) to the opposing edge (i.e., an edge furthest from thethermalizing point) in order to provide a sufficient temperaturegradient for improving the chances of a successful progression ofexpulsion of magnetic field along the cooling path during coolingthrough the critical temperature of planar shields 110 and 111 by coldsurface 140.

Planar shields 110 and 111 may, for example, each have a criticaltemperature higher than that of superconducting chip 120 so that amagnetic field is substantially expelled from the region in whichsuperconducting chip 120 is located before chip 120 is cooled throughits critical temperature. Superconducting chip 120 may, for example, bemade from materials such as Al and/or Nb and each of planar shields 110and 111 may, for example, consist of a bulk sheet of NbTi or a thinlayer of NbTi deposited onto a substrate. The NbTi layer's thickness maybe adjusted/formed to achieve a desired thermal resistance per unitlength from the respective thermalized corner of each of planar shields110 and 111 to their respective opposite edge. Together with the heatcapacitance per unit length of the substrate, the total heat capacitanceper unit length may passively provide for a sufficient/optimizedtemperature gradient for a successful progression of magnetic fieldexpulsion away from the respective thermalizing point of each of planarshields 110 and 111 while cooling through their respective criticaltemperatures without the need for controlled heating.

Planar shields 110 and 111 may, for example, each have the same or alower critical temperature than superconducting chip 120. The sampleholder 130 which holds superconducting chip 120 and coolssuperconducting chip 120 may be thermally coupled to a first coldsurface 140 that is cooled by a first cooling device (e.g., a firststage of a first cryogenic refrigeration system, not shown), but planarshields 110 and 111 may be thermally coupled to a second cold surface(not shown) that is itself thermally coupled to a second cooling device(e.g., a second stage of the first cryogenic refrigeration system, or asecond cryogenic refrigeration system, not shown). The second coolingdevice may be operated to cool the second cold surface faster or beforethe first cooling device cools the first cold surface 140, therebyenabling planar shields 110 and 111 to cool through their respectivecritical temperatures before superconducting chip 120 cools through itscritical temperature. In this exemplary configuration, planar shields110 and 111 may be made from the same material as superconducting chip120 or any other suitable material or a combination of materials thathave the same critical temperature as superconducting chip 120 or acritical temperature lower than that of superconducting chip 120,provided the planar shields 110 and 111 cool through their respectivecritical temperatures and become superconducting before superconductingchip 120.

The maximum field that can be applied to a superconductor at aparticular temperature while still permitting the superconductor tomaintain its superconductivity is the maximum critical field of thatsuperconductor. In accordance with the present systems, methods andapparatus, the maximum critical field of planar shields 110 and 111 maybe larger than the background/ambient field. In order to facilitatethis, the various embodiments of magnetic shielding systems describedherein may be combined with other magnetic shielding systems to ensurethe background/ambient field is lower than the maximum critical field ofplanar shields 110 and 111. As previously described, thermalizing points160 and 161 of respective planar shields 110 and 111 may be aligned sothat planar shields 110 and 111 may cool and become superconducting atleast approximately simultaneously and in substantially the samedirection, thereby minimizing the magnetic field intensity at thetransition boundary as it spreads across the respective surfaces ofplanar shields 110 and 111 during cooling through their respectivecritical temperatures. In this way, magnetic flux is expelled or pushedby the transition boundary towards the respective edge 110 a and 111 aof each of planar shields 110 and 111 that is opposite the respectivethermalizing corner. Edge 110 a of planar shield 110 is opposite (i.e.,further away from) thermalizing point 160 and edge 111 a of planarshield 111 is opposite (i.e., furthest away from) thermalizing point161. However, such may cause magnetic flux to accumulate at respectiveedges 110 a and 111 a of each of planar shields 110 and 111, which couldproduce a localized magnetic field that exceeds the maximum criticalfield of planar shield 110 and/or planar shield 111. Thus, employingother magnetic shielding systems (i.e., the cylinder approach) tominimize the ambient magnetic field may help to ensure that theaccumulated magnetic flux does not amount to a magnetic field exceedingthe maximum critical field of planar shields 110 and 111.

The separation distance between planar shields 110 and 111 andsuperconducting chip 120 has been enlarged in FIG. 1 to reduce clutter.In accordance with the present systems, methods and apparatus theshielding provided by magnetic shielding system 100 may be improved bypositioning planar shields 110 and 111 as close as possible tosuperconducting chip 120 while not touching any of superconducting chip120 or sample holder 130. For example, the separation distance betweenplanar shields 110 and 111 may be less than the length of each shield110 and 111 as well as the width of each shield 110 and 111. If planarshields 110 and 111 had a larger separation distance, depending on thedirection of the magnetic field, either the magnetic field that bendsaround shield 110 may tend to flow through superconducting chip 120before it bends around shield 111 or the magnetic field that bendsaround shield 111 may tend to flow through superconducting chip 120before it bends around shield 110. By placing the sheets as close aspossible, depending on the direction, either the magnetic field thatbends around shield 110 may be forced to bend around shield 111immediately after thereby preventing the magnetic field that was bentaround shield 110 from flowing through superconducting chip 120, or themagnetic field that bends around shield 111 may be forced to bend aroundshield 110 immediately after thereby preventing the magnetic field thatwas bent around shield 111 from flowing through superconducting chip120.

Throughout the remainder of this specification, various embodiments ofthe present systems, methods and apparatus are described that use asuperconducting processor chip to represent a system for which magneticshielding is desired. While the present systems, method and apparatusare well-suited for shielding a superconducting processor chip, those ofskill in the art will appreciate that other embodiments may be used toshield systems other than superconducting processor chips.

By using system 100, magnetic field present in the local environment ofsuperconducting chip 120 (for example, the magnetic field flowing normalto superconducting chip 120) may be reduced to a level that enablesand/or improves the operation of superconducting chip 120. In someapplications, system 100 may be particularly well-suited to reduce aspecific component of a magnetic field, such as the component of themagnetic field that is normal to planar shields 110 and 111 and, forexample, also normal to superconducting chip 120. As previouslydiscussed, those of skill in the art will appreciate that planar shields110 and 111 may be triangular in geometry, but need not be triangular ingeometry in order to realize at least some of the benefits taught in thepresent systems, methods and apparatus. Planar shields 110 and 111 maytake on any shape defined by a perimeter to encompass any desired planarregion; however, in general it is advantageous to ensure that planarshields 110 and 111 are both larger in planar area than the planar areaof superconducting chip 120 so that the planar area of superconductingchip 120 is completely covered by planar shields 110 and 111 which lieparallel to the plane of superconducting chip 120.

FIG. 2 is a side view of a magnetic shielding system 200 employingplanar shields 210 and 211 in accordance with the present systems,methods and apparatus. System 200 functions in a similar way to system100, except that system 200 clearly shows the way in which planarshields 210 and 211, a superconducting chip 220 and a sample holder 230are attached to a cold surface 240.

Thermally conductive leads 250 and 251 thermally couple planar shields210 and 211, respectively, to cold surface 240 and may be, for example,soldered to respective surfaces of planar shields 210 and 211. Those ofskill in the art will appreciate that thermally conductive leads 250 and251 may be attached to planar shields 210 and 211 in many alternativeways, such as by bolting or epoxy gluing. Non-superconducting solder maybe used, since the purpose here is thermal conduction rather thanelectrical, and the solder provides a nice large contact surface areabetween the led and the shield. Conductive lead 250 is clamped to coldsurface 240 with clamps 290 and 291. Conductive lead 251 is clamped tocold surface 240 with clamps 294 and 295. Sample holder 230 is clampedto cold surface 240 with clamps 292 and 293. Superconducting chip 220 ismounted on sample holder 230 with electrical wirebonds (not shown). Coldsurface 240 is attached to cooling device 241 which serves as the sourceof the cooling power in magnetic shielding system 200. The direction ofthe component of the magnetic field that may have the most effect onsuperconducting chip 220 is shown by arrow 201, which a person of skillin the art will appreciate is normal to the planar surface of chip 220.

Unlike the previously described “cylinder approach”, the presentsystems, methods and apparatus may easily be heated to further helpminimize the chance of trapping flux as shields 210 and 211 cool throughtheir respective critical temperatures and help control the expulsion ofmagnetic flux away from the thermalizing points by actively steepeningthe temperature gradient. This capability may be desired to accommodateirregularities on the surface of planar shields 210 and 211 that canlead to a not-so-smooth transition of temperature gradient away from thethermalizing points of planar shields 210 and 211 that can ultimatelyresult in trapped/pinned flux.

FIG. 3 shows a magnetic shielding system 300 that includes controllableheating elements 370 and 371 to further help minimize the chance oftrapping flux as planar shields 310 and 311 cool through theirrespective critical temperatures and help control the expulsion ofmagnetic flux away from the thermalizing points by actively steepeningthe temperature gradient. System 300 functions in a similar way tosystem 100 from FIG. 1 and system 200 from FIG. 2, except that system300 includes heating elements 370 and 371 (hereafter, “heaters”) whichmay be coupled to a controller 380. Controller 380 may, for example, becontrolled by an electrical current. Heaters 370 and 371 may beintegrated into system 300 to heat planar shields 310 and 311,respectively. Once the desired level of expulsion of the magnetic fieldhas been achieved by planar shields 310 and 311, heaters 370 and 371 maybe deactivated. This process can be repeated with varying heatersettings to optimize the results. The magnetic fields may be measuredusing magnetometer 321. Those of skill in the art will appreciate thatheaters 370 and 371 may be realized by a variety of hardware devices,including but not limited to resistors conductively or radiativelycoupled to planar shields 310 and 311 or light-emitting diodesprojecting photons onto the surfaces of planar shields 310 and 311. Thepurpose of heaters 370 and 371 is to control the thermal gradient acrossplanar shields 310 and 311 and thereby control the expulsion of magneticflux from superconducting chip 320, as illustrated in FIG. 4.

FIG. 4 is a top view of a magnetic shielding system 400 showing acontrolled temperature gradient (represented by arrow 410 c) of a planarshield 410 in accordance with the present systems, methods andapparatus. Arrow 410 c indicates the direction of the temperaturegradient from cold to warm across planar shield 410 (which is analogousto, e.g., planar shield 310 or 311 from FIG. 3, planar shield 210 or 211from FIG. 2, and/or planar shield 110 or 111 from FIG. 1) with athermally conductive lead 450 attached to a corner of shield 410 atthermalizing point 460 to cool shield 410 and a heater 470 integratedinto system 400 to supply a constant temperature along edge 410 a ofshield 410. Thermally conductive lead 450 may be elsewhere attached to acold surface (not shown) which may be thermally coupled to a coolingdevice (not shown) such as a cryogenic refrigeration system. Heater 470may be coupled to a controller (not shown) that is controlled by anelectrical current.

FIG. 5A shows an exemplary plot 500 a of the temperature along dottedline 410 b bisecting planar shield 410 from FIG. 4. The x-axis in FIG.5A indicates the position along line 410 b (from left to right) and they-axis in FIG. 5A indicates the corresponding temperature at eachposition along line 410 b. The values of both x- and y-axes in FIG. 5Amay depend on the actual physical parameters of the magnetic shieldingsystem being implemented. The purpose of FIG. 5A is to show the generalshape of line 590 a representing the temperature profile along line 410b from FIG. 4, i.e., that the temperature of planar shield 400 increasesalong line 410 b in a direction from right to left (or, alternatively,decreases along line 410 b in a direction from left to right). Planarshield 410 in FIG. 4 is at an instance of the cooling process, thecooling provided by thermally conductive lead 450 at thermalizing point460, with controlled heating provided by heater 470 at edge/end 410 awhich is furthest from thermalizing point 460 (i.e., controlled heatingat the left of FIG. 5A and controllable cooling at the right of FIG.5A). As seen in FIG. 5A, the temperature of planar shield 410 is lowestat thermalizing point 460 and the temperature gradually increasestowards edge 410 a. Therefore, thermalizing point 460 of planar shield410 will transition through the critical temperature and becomesuperconducting before edge 410 a of planar shield 410. The“superconductivity” may then gradually spread over to edge 410 a as therest of planar shield 410 cools through its critical temperature. Thoseof skill in the art will appreciate that irregularities on the surfaceof planar shield 410 may hinder the smooth progression ofsuperconductivity along the length of planar shield 410. FIG. 5A isgiven as an example only to show the temperature along shield 410 at aspecific instance. The shape of plot 500 a may be different at differentinstances of the cooling and heating process of shield 410.

FIG. 5B shows plot 500 b of the temperature gradient of FIG. 5A whichshows the temperature along dotted line 410 b from FIG. 4. The x-axis inFIG. 5B indicates the position along line 410 b (from left to right) andthe y-axis in FIG. 5B indicates the corresponding temperature gradientat each position along line 410 b. The values of both x- and y-axes inFIG. 5B may depend on the actual physical parameters of the magneticshielding system being implemented. The purpose of FIG. 5B is to showthe general shape of line 590 b representing the temperature gradientalong line 410 b from FIG. 4, taken at the same instance as FIG. 5A,i.e., that the temperature gradient of planar shield 410 increases alongline 410 b in a direction from left to right (or, alternatively,decreases along line 410 b in a direction from right to left). Theplanar shield 410 in FIG. 4 is at a specific instance of the coolingprocess with controlled heating at edge/end 410 a which is furthest fromthermalizing point 460 (i.e., controlled heating at the left of FIG. 5Band controlled cooling at the right of FIG. 5B). The steep slope of line590 b to the right along the x-axis of FIG. 5B indicates that thetemperature of planar shield 400 is rapidly changing around thermalizingpoint 460. The change in temperature may tend to decrease towards edge410 a, as indicated by the reduction in slope, or leveling off, of line590 b to the left along the x-axis. For some applications, it may beimportant to achieve a smooth temperature gradient as shown in FIG. 5Bas otherwise planar shield 410 may trap the unwanted magnetic field. Theshape of plot 500 b may be different at different instances of thecooling and heating process of shield 410.

Certain aspects of the present systems and methods may be realized atroom temperature, and certain aspects may be realized at asuperconducting temperature. Thus, throughout this specification and theappended claims, the term “superconducting” when used to describe aphysical structure such as a “superconducting sheet” or a“superconducting chip” is used to indicate a material that is capable ofbehaving as a superconductor at an appropriate temperature. Asuperconducting material may not necessarily be acting as asuperconductor at all times in all embodiments of the present systems,methods and apparatus. As previously described, the planar shields(e.g., shields 110 and 111) may each have an area larger than thesuperconducting chip (e.g., chip 120) or any other device beingshielded, and one planar shield (e.g., shield 110) may be positionedproximate a first surface of the superconducting chip while the otherplanar shield (e.g., shield 111) may be positioned proximate a secondsurface of the superconducting chip, where the second surface of thesuperconducting chip is opposite the first surface of thesuperconducting chip, thereby sandwiching the superconducting chip.Throughout this specification and the appended claims, the phrase “areaof the planar shield(s)” or “planar area of the shield(s)” refers to thearea taken along the two largest dimensions spanned by the planar shield(e.g., the length and width of the shield in the two-dimensional planein which the shield lies, as opposed to the thickness of the shield)that may form a plane parallel to the plane of the superconducting chip.

The two planar shields may be similar in shape and area. Those of skillin the art will appreciate that the term “similar shape” is used togenerally encompass implementations in which the shape of one planarshield may be slightly different from the shape of the other planarshield and the term “similar area” is used to generally encompassimplementations in which the area of one planar shield may be slightlydifferent from the area of the other planar shield. As long as the twoplanar shields completely cover the planar area of the superconductingchip so that the shields may shield the superconducting chip from thecomponent of the magnetic field that is normal to the superconductingchip plane and the planar shields expel flux away from thesuperconducting chip, the planar shields may have any area and anyshape.

The planar shields may be positioned parallel to each other as well asto the device being shielded (i.e., if the device itself is planar, asin the case of a superconducting chip). Those of skill in the art willappreciate that the term “parallel” is used loosely in this context andthe planar shields and the device being shielded may be slightlyoff-parallel.

A planar shield may be formed from a single continuous sheet ofsuperconducting material or from multiple discontinuous sheets ofsuperconducting material that are thermally and/or electricallyconnected together to form a single superconducting sheet.

The various embodiments described herein may be combined with otherforms of magnetic shielding systems, including but not limited to thecylindrical approach to magnetic shielding.

At least one planar shield may be physically coupled to asuperconducting processor chip, though preferably through a thermallyinsulative material.

The above description of illustrated embodiments, including what isdescribed in the Abstract, is not intended to be exhaustive or to limitthe embodiments to the precise forms disclosed. Although specificembodiments of and examples are described herein for illustrativepurposes, various equivalent modifications can be made without departingfrom the spirit and scope of the disclosure, as will be recognized bythose skilled in the relevant art. The teachings provided herein of thevarious embodiments can be applied to other systems, methods andapparatus, not necessarily the exemplary systems, methods and apparatusgenerally described above.

The various embodiments described above can be combined to providefurther embodiments. All of the U.S. patents, U.S. patent applicationpublications, U.S. patent applications, foreign patents, foreign patentapplications and non-patent publications referred to in thisspecification and/or listed in the Application Data Sheet areincorporated herein by reference, in their entirety. Aspects of theembodiments can be modified, if necessary, to employ systems, circuitsand concepts of the various patents, applications and publications toprovide yet further embodiments.

These and other changes can be made to the embodiments in light of theabove-detailed description. In general, in the following claims, theterms used should not be construed to limit the claims to the specificembodiments disclosed in the specification and the claims, but should beconstrued to include all possible embodiments along with the full scopeof equivalents to which such claims are entitled. Accordingly, theclaims are not limited by the disclosure.

1. A superconducting magnetic shielding system operable to shield adevice from a component of a magnetic field, the device having at leasta first surface and a second surface that is opposite the first surface,the system comprising: a first planar shield formed of a material thatis superconducting below a critical temperature, the first planar shielddefined by a perimeter and an area that is larger than an area of thedevice; a second planar shield formed of a material that issuperconducting below a critical temperature, the second planar shielddefined by a perimeter and an area that is larger than the area of thedevice, wherein the first planar shield and the second planar shield areparallel with one another, the first planar shield is proximate thefirst surface of the device, the second planar shield is proximate thesecond surface of the device, and wherein the first and second planarshields are both oriented normal to the component of the magnetic field;a first thermally conductive lead that is thermally coupled to a pointproximate the perimeter of the first planar shield and operable tothermally couple to a cold surface; and a second thermally conductivelead that is thermally coupled to a point proximate the perimeter of thesecond planar shield and operable to thermally couple to the coldsurface.
 2. The magnetic shielding system of claim 1 wherein the pointproximate the perimeter of the first planar shield is aligned with thepoint proximate the perimeter of the second planar shield.
 3. Themagnetic shielding system of claim 1 wherein the first and second planarshields are positioned such that the device is proximate a center of thefirst planar shield and a center of the second planar shield.
 4. Themagnetic shielding system of claim 1 wherein a shape of the first planarshield is similar to a shape of the second planar shield.
 5. Themagnetic shielding system of claim 4 wherein the shape of the firstplanar shield is triangular and the shape of the second planar shield istriangular.
 6. The magnetic shielding system of claim 4 wherein the areaof the first planar shield is similar to the area of the second planarshield.
 7. The magnetic shielding system of claim 1 wherein the deviceincludes a material that is superconducting below a criticaltemperature.
 8. The magnetic shielding system of claim 7 wherein thecritical temperature of the first planar shield is higher than thecritical temperature of the device and the critical temperature of thesecond planar shield is higher than the critical temperature of thedevice.
 9. The magnetic shielding system of claim 8 wherein the firstplanar shield and the second planar shield are both formed of a materialselected from the group consisting of: niobium, tin, aluminum, lead,bulk niobium-titanium alloy, or a layer of niobium-titanium alloydeposited onto a substrate.
 10. The magnetic shielding system of claim 7wherein the device includes niobium.
 11. The magnetic shielding systemof claim 7 wherein the device is a superconducting chip including asuperconducting processor.
 12. The magnetic shielding system of claim 11wherein the superconducting processor includes a superconducting quantumprocessor.
 13. The magnetic shielding system of claim 1 wherein thedevice is planar.
 14. The magnetic shielding system of claim 14 whereinthe device is parallel to both the first planar shield and the secondplanar shield.
 15. The magnetic shielding system of claim 14 wherein aseparation distance between the first planar shield and the secondplanar shield is less than a length of the first planar shield, a widthof the first planar shield, a length of the second planar shield, and awidth of the second planar shield.
 16. The magnetic shielding system ofclaim 1 wherein the first thermally conductive lead and the secondthermally conductive lead both comprise copper.
 17. The magneticshielding system of claim 1 wherein a respective critical field of eachof the first planar shield and the second planar shield is higher than amaximum magnitude of the magnetic field.
 18. The magnetic shieldingsystem of claim 1, further comprising: a controllable heating devicecomprising a first heater and a second heater, wherein the first heateris proximate a first end of the first planar shield and the secondheater is proximate a first end of the second planar shield.
 19. Themagnetic shielding system of claim 18, wherein the first end of thefirst planar shield is a furthest end from the point proximate theperimeter of the first planar shield at which the first thermallyconductive lead is thermally coupled, and wherein the first end of thesecond planar shield is a furthest end from the point proximate theperimeter of the second planar shield at which the second thermallyconductive lead is thermally coupled.